OK, first are you happy with this one: we have f(x) = x^2 - 4x, what is f(a)?
(Answer: f(a) = a^2 - 4a. i.e. you just plug a into wherever x used to appear. It does NOT mean "f(a) = f(x) * a", which is similar to what you seem to have done in your attempt.)

Now, use the same technique for f(x+2). We just plug (x+2) into wherever x used to appear. What do you reckon the answer should be?

Once you have written f(x+2) as a polynomial, then we can call it g(x), or h(x) or y(x). So the "g(x) = " bit isn't that important.

OK, first are you happy with this one: we have f(x) = x^2 - 4x, what is f(a)?
(Answer: f(a) = a^2 - 4a. i.e. you just plug a into wherever x used to appear. It does NOT mean "f(a) = f(x) * a", which is similar to what you seem to have done in your attempt.)

Now, use the same technique for f(x+2). We just plug (x+2) into wherever x used to appear. What do you reckon the answer should be?

Once you have written f(x+2) as a polynomial, then we can call it g(x), or h(x) or y(x). So the "g(x) = " bit isn't that important.